1. Field of the Invention
The present invention relates to a viscous clutch or coupling assembly adapted for installation in torque transmission systems in a motor vehicle for effecting a drive connection between a pair of rotary members coaxially arranged for relative rotation.
2. Discussion of the Prior Art
Conventional viscous coupling assemblies of this kind are classified, in general, into two types. Disclosed in Japanese Patent Laid-open Publication No. 61-501583 is a viscous coupling assembly of the first type which is adapted to automatically effect torque transmission between drive and driven rotary members in response to relative rotation thereof. Disclosed in Japanese Patent Laid-open Publication No. 63-287631 is a viscous coupling assembly of the second type which is adapted as a limited-slip differential to restrict relative rotation between drive and driven rotary members, between a pair of drive rotary members or between a pair of driven rotary members. The viscous coupling assembly of the first type is mainly installed in one of torque transmission systems in a four-wheel drive vehicle of the real-time type. The coupling assembly of the second type is mainly installed in a differential unit.
In the viscous coupling assembly of the first type, torque transmission is effected by shearing force of viscous fluid between the rotary members. Viscous frictional torque caused by the shearing force, however, does not increase so large, and the rising characteristic of viscous frictional torque is not so good. In case the viscous coupling assembly is constructed to generate a large amount of viscous frictional torque, the shearing force is noticeably increased to increase the heating value of viscous fluid. As a result, the viscosity of fluid is greatly lowered to cause fluctuation of the viscous frictional torque. For this reason, there will occur hysteresis in the torque transmission characteristic of the coupling assembly when the difference in rotation between the rotary members is gradually increased and decreased. In the case that the viscous coupling assembly is installed in one of the torque transmission systems or installed as a limited-slip differential in the differential unit, transmission torque or limited-slip torque becomes unstable. This causes undesired influence to running performance of the vehicle.
The viscous coupling assembly of the second type includes a set of rotary members mounted therein for relative rotation to generate viscous frictional torque for torque transmission, cam means cooperable with the rotary members to convert the viscous frictional torque into a thrust force, and a friction clutch cooperable with the cam means to effect the torque transmission when engaged by the thrust force applied thereto from the cam means. Since the rotary members act substantially in the same manner as those in the coupling assembly of the first type, the problems discussed above will remain.
In copending U.S. patent application Ser. No. 07/505,478, filed on Apr. 6, 1990, there has been proposed a viscous clutch assembly the construction of which is designed to solve the problems discussed above. The viscous clutch assembly comprises first means for generating a viscous resistance force in response to relative rotation between a pair of coaxially arranged rotary members, second means cooperable with the first means for converting the viscous resistance force into a thrust force, third means in the form of a clutch mechanism cooperable with the second means to be brought into engagement when applied with the thrust force from the second means for establishing a drive connection between the rotary members, and fourth means cooperable with the first means for increasing and decreasing the viscous resistance force in accordance with a difference in torque between the rotary members.
In such an arrangement of the viscous clutch assembly, the first means is in the form of a viscous coupling which includes a rotary plate arranged for rotation with one of the rotary members and a first cam member rotatably mounted on the one of the rotary members for relative rotation with the rotary plate, the rotary plate being formed at one side thereof with a first set of concentrically spaced annular fins which are coupled with a second set of concentrically spaced annular fins formed on one side of the first cam member through a predetermined quantity of viscous fluid stored between the rotary plate and the first cam member, and the second means is in the form of a cam mechanism which includes a second cam member arranged for rotation with the other rotary member and cooperable with the first cam member for generating a thrust force in accordance with a viscous resistance force acting on the first cam member during relative rotation between the rotary plate and the first cam member.
In the viscous clutch assembly, shearing speed of the viscous fluid increases in accordance with an increase of the difference in rotation between the rotary members, resulting deterioration of the viscosity of fluid. As a result, the increasing rate of transmission torque is lowered in accordance with an increase of the difference in rotation between the rotary members as shown by a characteristic curve I in FIG. 11. Since the characteristic curve I is gradually spaced from a region III where phenomenon of tight corner braking occurs, it becomes difficult to enhance running performance of the vehicle.